Method of and system for detecting a serial arc fault in a power circuit

ABSTRACT

A method of detecting a serial arc fault in a DC-power circuit includes injecting an RF-signal with a narrow band-width into the DC-power circuit and measuring a response signal related to the injected RF-signal in the DC-power circuit. The method further includes determining a time derivative of the response signal, analyzing the time derivative, and signaling an occurrence of a serial arc fault in the power circuit based on the results of the analysis. A system for detecting an arc fault is configured to perform a method as described before.

FIELD

The disclosure relates to a method of detecting a serial arc fault in apower circuit, in particular within a photovoltaic system. Thedisclosure further relates to an arc fault detection system and aphotovoltaic system comprising a corresponding detection system.

BACKGROUND

Power circuits, in particular DC (Direct Current)-power circuits thatwork with high voltages and high currents, for example in photovoltaicsystems or systems providing off-grid power supply, are prone to thedevelopment of electric arcs. Electric arcs can, for example occur whena power line with a high current load is interrupted during maintenanceor in case contacts at interconnectors are degraded. Other possiblecauses of arc faults, i.e. the occurrence of an electric arc in thepower circuit, are corroded solder joints or broken insulators of thepower lines. Arc faults are the most common fire causes in photovoltaicsystems. This also reflects in the requirements for arc fault protectionfor photovoltaic systems as for example regulated by the NationalElectric Code (NEC) 690.11 of the United States of America.

A reliable arc fault detection method and system is therefore of majorimportance. On the one hand, for security reasons the existence of anarc fault has to be detected with a reliability as high as possible. Onthe other hand, the probability of an erroneous indication of a supposedarc fault has to be as low as possible, in particular, if an erroneousdetection of an arc fault might lead to a shutdown of a photovoltaicsystem without the option to automatically restart it, as for examplespecified in the before mentioned NEC 690.11 code.

Electric arcs usually emit a broadband AC (Alternating Current)-signalin an RF (Radio Frequency)-frequency range. Detection systems for arcfaults that are based on detecting a radio frequency signal in the powercircuit are well established and, for example, known from document WO95/25374. A problem associated with the detection of arc faults viatheir AC-current signature can arise from the weakness of the signal. Inorder to ensure that arc faults are assuredly detected, detectioncircuits with a high detection sensitivity are required. A down-side ofhigh detection sensitivity could be an increased number of nuisancealarm situations, in which a noise signal, e.g. due to a disturbingRF-signal that is coupled into the detection circuit, is misinterpretedand wrongly assigned to the presence of an arc fault. Possible sourcesof disturbing signals are for example RF-radio transmitter, electrictrains or trams passing by, electric or electronic devices with aninsufficient electro-magnetic shielding or interference suppression, orarcs in adjacent power systems.

Document US 2014/062500 A1 describes a further development of thedetection of arcs based on RF-signals emitted by the arc. According tothis document, an RF-signal is actively injected into the DC-powercircuit and its frequency response is measured and analyzed. Thesemethod steps allow for an identification of a preferred frequency rangewhich is afterwards used to detect the signal emitted by a burning arc.Albeit the detection is made more reliable, still high detectionsensitivity is needed to be able to detect the RF-signal emitted by anarc. The vulnerability for nuisance alarm situations therefore persists.

It is thus desirable to create a method and system for reliablydetecting arc faults in a power circuit. It is furthermore desirable todescribe a photovoltaic system with a corresponding detection system.

SUMMARY

According to one embodiment of the disclosure, a method of detecting aserial arc fault in a DC-power circuit comprises injecting an RF-signalwith a narrow band-width into the DC-power circuit and measuring aresponse signal related to the injected RF-signal in the DC-powercircuit. The method further comprises determining a time derivative ofthe response signal, analyzing the time derivative, and signaling anoccurrence of a serial arc fault in the power circuit based on theresults of the analysis.

By measuring a response signal to an injected RF-signal rather than anRF-signal generated by an arc itself, the measurements are performed ata much higher signal level, thereby minimizing the susceptibility tonuisance alarm events. The detection via the time derivative furtherincreases the reliability of the method since comparable changes in theresponse signal do not occur under normal operation conditions.

In one embodiment of the method, the acts are performed repeatedly inorder to ensure continuous observation.

In another embodiment of the method, the injected RF-signal comprises asine wave at a single frequency. If the injected signal has a verynarrow band width due to the use of a single sine-wave, the responsesignal can be selectively amplified and observed with a highsignal-to-noise ratio. Filters and/or PLL (phase locked loop) techniquescan, for example, be used to discriminate the signal from backgroundnoise.

In a further embodiment of the method, the frequency of the RF-signal ischosen from a frequency range where an impedance of the DC-power circuitonly shows a small dependency on operating conditions of the DC-powercircuit. This way operation of the DC-power circuit does not interferewith or disturb the arc detection. If, for example, the DC-power circuitis the DC-power circuit of a photovoltaic (PV) system, a change of thesolar irradiation and a following change of the point of operation of aPV-generator in the system would not lead to a change in the RF-responsesignal that could be interpreted as an occurrence of an arc. In oneembodiment, the RF-signal has a frequency of at least 100 kilohertz(kHz). In one embodiment the RF-signal is 150 kHz+/−1 kHz. It is found,in particular in PV-systems, that the before mentioned condition (smalldependency of impedance on operating conditions) is fulfilled forfrequencies above 100 kHz.

In another embodiment of the method, the occurrence of an arc fault issignaled if the time derivative of the response signal exceeds apredetermined threshold. In a further embodiment, the occurrence of anarc fault is signaled if the time derivative of the response signalexceeds a predetermined threshold and if the response signal showsstatistical fluctuations above a certain level afterwards. An amplitudeof the statistical fluctuations could be determined and compared to afurther threshold to evaluate whether the statistical fluctuations areabove a certain level. The RF-response signal shows characteristicfluctuations during the presence of an electric arc. Observing thesefluctuations after the (presumable) detection of an arc can be seen as aproof for correct arc detection. Accordingly, combining arc detection byobserving rapid changes of the measured RF-signal and confirming thisdetected arc by observing fluctuations of the measured RF-signal on ashort time scale provides an arc detection method with an even higherreliability.

According to another embodiment of the disclosure, an arc faultdetection system comprises an injection circuit or means configured toinject an RF-signal into the DC-power circuit, an outcoupling circuit ormeans configured to measure an RF-response signal related to anRF-current flowing in the power circuit, and a control circuit operablyconnected to the outcoupling circuit or means. The arc fault detectionsystem is configured to perform a method according to the disclosure.The same advantages as described in connection with the method arise.

In one embodiment of the arc fault detection system the injectioncircuit or means and/or the outcoupling circuit or means areadditionally used for PLC-data transfer. PLC (power line communication)is a known method to transfer data in DC-power circuits. In PV-systems,for example, PLC can be used to exchange information between an inverterand a PV-generator, e.g. for control purposes. By combining the arcfault detection system with a PLC-system, components can be shared byboth applications and the total number of components can be reduced.

According to another embodiment of the disclosure, a power system, inparticular a photovoltaic power system, comprises an arc fault detectionsystem according to the disclosure. The same advantages as described inconnection with the method and arc fault detection system arise.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is now described in more detail and will be fullyunderstood with reference to the following detailed description inconjunction with the drawings. The drawings show:

FIG. 1 a schematic wiring diagram of a photovoltaic system with an arcfault detection system;

FIG. 2 a flow chart of an embodiment of a method for detecting thepresence of an electric arc in a power circuit; and

FIG. 3 illustrates an example of a time dependence of a measuredRF-response signal in a photovoltaic system.

DETAILED DESCRIPTION

FIG. 1 shows a photovoltaic system 1, in the following abbreviated asPV-system 1, in a schematic wiring diagram as an example of a DC (DirectCurrent)-power circuit. The PV-system 1 comprises a photovoltaicgenerator 2 (PV-generator 2) that is connected to an inverter 5, e.g. aDC/AC (Direct Current/Alternating Current)-converter, by way of DC-powerlines 3, 4. The inverter 5 is connected to a power grid 6 on itsAC-side.

The power grid 6 can either be a private or a public power grid. By wayof example, the power grid 6 is a 3-phase system and the inverter 5 isdesigned to feed in all 3-phases. However, the disclosure can berealized with a power grid and/or inverter operating with any number ofphases, for example, one or two phases.

Also by way of example, the PV-generator 2 is symbolized by the circuitsymbol of a single photovoltaic cell. In a realization of the shownPV-system 1, the PV-generator 2 can, for example, be a singlephotovoltaic module (PV-module) that itself comprises a plurality ofphotovoltaic cells. In another embodiment, the PV-generator 2 cancomprise a plurality of PV-modules that, for example, are connected inseries and form a so-called string. Furthermore, a parallel connectionor a combined serial/parallel connection of PV-modules is possible.

A possible serial arc fault 7 that can occur in the power circuit formedby the PV-generator 2, the power lines 3, 4 and the input stage of theinverter 5 is depicted in FIG. 1. The serial arc 7 is electrically inseries with the PV-generator 2 and is, by way of example, located inpower line 3. The serial arc 7 could also be located in power line 4 orwithin the PV-generator 2 or at a connection of the power lines 3, 4with the PV-generator 2 and/or with the inverter 5.

The PV-system 1 of FIG. 1 is equipped with an arc fault detection system10 for detecting serial arcs, e.g. the shown serial arc 7. The arc faultdetection system 10 comprises signal injection circuit or means 11 forproviding an RF (radio frequency)-signal that is injected into theDC-power circuit of the PV-system 1. In the schematic diagram of FIG. 1the signal injection circuit or means 11 is connected in series with theother components of the DC-power circuit and therefore carries aDC-current flowing in the DC-power circuit. The signal injection circuitor means 11 can e.g. comprise a signal generator circuit and anRF-transformer with a primary side and a secondary side. The primaryside of the transformer is connected to the signal generator circuit.The secondary side of the transformer has a low DC-resistance and islooped into the DC-power circuit.

The arc fault detection system 10 further comprises an outcouplingcircuit or means 12 for coupling out an RF-signal from the DC-powercircuit. The outcoupling circuit or means can e.g. again be realized bya transformer with a primary side and a secondary side. In this case,the primary side of the transformer shows a low DC-resistance and islooped into the power circuit. It would also be possible to use aresistor (“shunt”) or another component with a defined impedance as theoutcoupling circuit or means 12. In further embodiments, otherknown-types of outcoupling circuits or means can be used, for exampleHall sensors. The purpose of the outcoupling circuit or means 12 is tocouple an RF-signal out of the DC-power circuit to enable an analysis ofan RF-current flowing in the DC-power circuit.

It is noted that all-known circuits or means that can be used to eitherinject an AC-signal into a high current DC-circuit and/or to pick up anRF-signal component present in a DC-circuit can be utilized inconnection with the signal injection circuit or means 11 and/or theoutcoupling circuit or means 12. In particular, the outcoupling circuitor means 12 can be realized as a pickup-coil which is assigned to therespective DC-power line 3, 4. The pickup-coil can, for example, beconfigured as a Rogowski-coil. The primary side of the outcouplingcircuit or means 12 then could be a cable of the DC-power line 3, 4 or aprinted circuit board track within the inverter 5.

The arc fault detection system 10 further comprises a control circuit 13that controls the signal injection circuit or means 11 and analyzes theRF-signal provided by the outcoupling circuit or means 12. Control ofthe signal injection circuit or means 11 might comprise controlling asignal amplitude and/or frequency of the RF-signal injected into theDC-power circuit. The control circuit 13 provides an output signal line14 that signals a detected arc. It is noted that the presence of adedicated output signal line 14 is just one example embodiment. Anoutput signal could also be provided via a wired and/or wireless dataconnection.

In the embodiment shown in FIG. 1 the arc fault detection system 10 usesdedicated components like the signal injection circuit or means 11 andthe outcoupling circuit or means 12 exclusively for arc detection. Inanother embodiment the signal injection circuit or means 11 and theoutcoupling circuit or means 12 are also used for other purposes, inparticular for a data exchange over the DC-power lines 3, 4. Datatransfer via DC-power lines is known as PLC (power line communication).It is known to use PLC in PV-systems to exchange information between aninverter and a PV-generator, e.g. for control purposes. By combining thearc fault detection system 10 with a PLC-system, components can beshared by both applications and the total number of components can bereduced.

Details of a method for detecting an arc fault are described in thefollowing with reference to FIGS. 2 and 3.

FIG. 2 shows a flow chart of a method for detecting an electric arc in apower circuit. The method can, for example, be performed by a detectionsystem 10 as shown in FIG. 1. Without any limitation, it will thereforebe described by way of example with reference to FIG. 1. The method isparticularly suited for the detection of a serial arc, e.g. like theserial arc 7 shown in FIG. 1.

In a first act S1, the power circuit, for example the PV-system 1 shownin FIG. 1, starts its operation. In case of the PV-system 1 of FIG. 1,this for example regularly occurs in the morning, when the intensity ofthe sunlight increases and the electric power produced by thePV-generator 2 is sufficient to start operation of the inverter 5. Asmentioned before, in one embodiment the detection system 10 could beintegrated into the inverter 5. The functionality of the control circuit13 could then be provided by a central control circuit of the inverter5. Alternatively, a separate control circuit 13 could be used which istriggered to perform the following acts by the control circuit of theinverter 5 after the inverter 5 has started its operation and performsits internal set up procedures.

In a second act S2, an RF-signal is generated by a signal generatorcircuit and injected into the DC-power circuit via the injection circuitor means 11. In one embodiment of the example of FIG. 1, a signalgenerator circuit of the injection circuit or means 11 or associatedwith the injection circuit or means 11 generates a sine-signal of aparticular frequency with a single frequency component, i.e. a signalwith a narrow frequency spectrum. The signal can have a frequency in arange of a few tens of kilohertz (kHz) to several hundreds of kilohertz.In one embodiment, the frequency is higher than 100 kHz.

The RF-signal injected into the DC-power circuit leads to an RF-currentflowing in the DC-power circuit. In case an input-stage of the inverter5 has a high impedance for RF-signals, a capacitor 8, depicted by adashed-line in FIG. 1, might be connected in parallel to the input ofthe inverter 5. This capacitor allows an RF-current to flow in the powercircuit. Often, a suitable capacitor is already present in theinput-stage of an inverter, e.g. as part of an EMC (electromagneticcompatibility)-filter.

In an act S3 that is performed in parallel to act S2, a response signalwithin the power circuit that is related to the injected RF-signal ismeasured via the outcoupling circuit or means 12. The measurement itselfis performed in the control circuit 13 in the embodiment shown inFIG. 1. In alternative embodiments, the measurement can be performed byor supported by an additional circuit that e.g. comprises a (pre-)amplifier and/or filter. A measured voltage amplitude of the RF-signalcorresponds to an RF-current I_(s) flowing in the DC-power circuit.Given that the amplitude of the RF-signal in-coupled into the DC-powercircuit is constant, the RF-current I_(s) reflects an impedance of theDC-power circuit at the frequency of the RF-signal.

FIG. 3 shows an example of a time dependence of a measured RF-currentI_(s) in the DC-power circuit. The measured signal is shown in form of acurrent curve 20 in a diagram that depicts a root mean square(RMS)-value of the measured current signal I_(s) on a vertical axis as afunction of a time t measured in seconds on a horizontal axis. As anexample, 10 seconds of the continuously measured curve 20 are shown inthe diagram of FIG. 3.

From a time t=0 seconds (s) to approximately t=65 the measuredRF-current I_(s) is more or less constant with slight slow variations.Its value is around a first current value I₁. The slight variations inthe current I_(s) arise from a change of the working point, also namedpoint of operation, of the PV-generator 2. Working point variations stemfrom a change in the irradiation situation. The inverter 5 of theembodiment of FIG. 1 comprises a tracking mechanism that tries tooperate the photovoltaic generator 2 under all or at least mostcircumstances at its most efficient working point for a given solarirradiation. The fact that the change in the point of operation of thephotovoltaic generation 2 reflects in the measured RF-current I_(s) isdue to the fact that the impedance of the DC-power circuit slightlychanges with a change of the point of operation. The influence of thepoint of operation can be minimized by choosing a signal frequency in afrequency range where this influence is less pronounced. Investigationsshow that the influence usually becomes smaller at higher frequencies,in particular at frequencies above 100 kHz.

At a time t* an electric arc evolved in the DC-power circuit. Theevolution of the arc leads to an immediate drop of the impedance in theDC-power circuit, which results in an according drop of the measuredRF-current I_(s). In the diagram of FIG. 3 the RF current I_(s) dropsfrom the first current value I₁ to a second current value I₂ that isalmost 40% smaller than the first current value I₁. At t>t* the measuredRF-current I_(s) keeps its mean value around this second current valueI₂. Due to the stochastic processes in a burning arc the RF-currentI_(s) is less stable than it was before the arc evolved. This results incurrent fluctuations of the RF-current I_(s) that occur on a relativelyshort time scale and in particular on a time scale that is much shorterthan the variations due to the tracking mechanism that was observed fort<t*.

According to one embodiment of the present disclosure, the rapid changein the measured RF-current I_(s) is used to detect the evolution of anarc in the DC-power circuit.

Therefore, in a next act S4 of FIG. 2, the measured RF-current I_(s) isanalyzed and in particular its first time derivative is determined. Theabsolute value of the time derivative, i.e. |dI_(s)/dt|, is determined.

In a next act S5 the absolute value of the time derivative is comparedto a predefined threshold. If the time derivative of the measuredRF-current signal I_(s) exceeds the threshold, the method branches to anact S6, in which an arc fault is signalled. The signalling can, forexample, occur via the output line 14 in the embodiment of FIG. 1. Theoutput line 14 can be connected to a visual and/or acoustical alarmindicator informing about the detected occurrence of an electric arc inthe DC-power circuit. In the example shown the method finishes after thesignalling.

If the time derivative of the RF-current signal I_(s) did not exceed thethreshold at act S5, the method branches back to acts S2/S3, therebycontinuing the method.

In an alternative embodiment the method could further comprise an act ofextinguishing the detected arc, for example by advising the inverter 5of the photovoltaic system 1 to interrupt any DC-currents flowing in theDC-power circuit. An interruption of the DC-current will extinguish theburning serial electric arc 7. In a further act the method could waitfor a manual restart signal before operation would be resumed and themethod would start again at act S1 of the diagram of FIG. 2.

In the embodiment described before the sudden drop in the measuredRF-current I_(s) was used to determine the evolution of an electric arcin the DC-power circuit. As described before and as apparent from FIG.3, the current curve 20 shows characteristic fluctuations during thepresence of an electric arc. Observing these fluctuations after the(presumable) detection of an arc can be seen as a proof for correct arcdetection. Accordingly, combining arc detection by observing rapidchanges of the measured current and confirming this detected arc byobserving fluctuations of the measured current I_(s) on a short timescale provides an arc detection method with a very high reliability.

It is finally noted that the foregoing description and the drawings areexamples and not restrictive and that the disclosure is not limited tothe disclosed embodiments. Other variations to the disclosed embodimentscan be understood and effected by those skilled in the art from a studyof the drawings, the disclosure, and the appended claims.

1. A method of detecting a serial arc fault in a DC-power circuit, themethod comprising: a) injecting an RF-signal into the DC-power circuit;b) obtaining a response signal related to the injected RF-signal in theDC-power circuit; c) determining a time derivative of the responsesignal; d) analyzing the time derivative of the response signal; and e)signaling an occurrence of a serial arc fault in the power circuitdepending on results of the analysis.
 2. The method of claim 1, whereinacts a) to e) are performed repeatedly.
 3. The method of claim 1,wherein the injected RF-signal comprises a sine wave at a singlefrequency.
 4. The method of claim 3, wherein a frequency of theRF-signal is chosen from a frequency range where an impedance of theDC-power circuit only shows a small dependency on operating conditionsof the DC-power circuit.
 5. The method of claim 3, wherein the RF-signalhas a frequency of at least 100 kHz.
 6. The method of claim 1, whereinthe occurrence of an arc fault is signaled if a time derivative of theresponse signal exceeds a predetermined threshold.
 7. The method ofclaim 1, wherein the occurrence of an arc fault is signaled if a timederivative of the response signal exceeds a predetermined threshold andif the response signal shows statistical fluctuations above apredetermined level.
 8. The method of claim 7, wherein an amplitude ofthe statistical fluctuations is determined and compared to a furtherthreshold to evaluate whether the statistical fluctuations are above apredetermined level.
 9. An arc fault detection system for a DC-powercircuit, comprising: an injection circuit configured to inject anRF-signal into the DC-power circuit; an outcoupling circuit configuredto extract an RF-response signal related to an RF-current flowing in theDC-power circuit; a control circuit operably connected to theoutcoupling means; wherein the control circuit is configured todetermine a time derivative of the RF-response signal, to evaluate thetime derivative, and to selectively signal an occurrence of an arc faultin the DC-power circuit based on the evaluated time derivative.
 10. Thearc fault detection system of claim 9, wherein the injection circuitcomprises a signal generator circuit configured to generate a sine waveat a single frequency.
 11. The arc fault detection system of claim 9,wherein the injection circuit and/or the outcoupling circuit areadditionally used for a PLC-data transfer.
 12. A method of detecting aserial arc fault in a DC-power circuit, comprising: injecting anRF-signal into the DC-power circuit; obtaining a response signal relatedto the injected RF-signal in the DC-power circuit; and selectivesignaling an occurrence of a serial arc fault in the DC-power circuitbased on the response signal.